1. Field of the Invention
The present invention relates to a stacker crane configured to perform a transfer function to retrieve or store work pieces from or into multiple racks.
2. Description of the Related Art
Stacker cranes have been used to retrieve or store workpieces from or into the storage shelves of racks installed in a warehouse. A stacker crane includes a platform configured to move vertically, and travels on tracks installed along racks, locates the platform near the desired one of storage shelves arranged vertically and horizontally in a rack, and retrieves or stores workpieces from or into the storage shelf using a transfer device mounted on the platform. There is known a type of stacker crane which includes a pair of masts disposed on a base configured to travel on tracks and a workpiece transfer device mounted on a platform configured to be raised or lowered while being guided by the masts, as disclosed in Japanese Unexamined Patent Application Publication No. 2008-74543 and Japanese Unexamined Patent Application Publication No. 2012-12152.
In the stacker cranes disclosed in Japanese Unexamined Patent Application Publication No. 2008-74543 and Japanese Unexamined Patent Application Publication No. 2012-12152, a platform is raised or lowered while being guided by a pair of masts, and linear motion guides are used as guiding parts. To restrain movement of the platform in directions other than the movement direction (vertical direction), the linear motion guides are formed by combining corrugated surfaces. By causing these corrugated surfaces to slide, the linearity of the movement is maintained. The platform guided by such linear motion guides does not wobble when raised or lowered and can stably carry work pieces. On the other hand, it has difficulty in increasing the elevation speed since it uses the sliding motion of the corrugated surfaces. As the sizes of warehouses have been increased in recent years, stacker cranes are required to increase the elevation speed of the platform. However, stacker cranes using linear motion guides have difficulty in increasing the elevation speed.
Conceivable configurations capable of increasing the elevation speed of the platform include one where a pair of guide rollers mounted on a platform sandwich a mast so that the platform is guided by the mast via the guide rollers. In this configuration, however, when the platform is tilted, the guide rollers may be detached from the mast. Thus, a portion of the platform may collide with the mast, resulting in damage to the mast, platform, or the like. Further, it takes time to recover from such a situation. Particularly, in a configuration where a platform is suspended from belts attached to a pair of masts, when the drive synchronization between the belts is lost, the platform is tilted. If the elevation speed is increased, the loss of the drive synchronization would increase the tilt of the platform.